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    Re: Hello (+ a few questions)
    From: Thomas Schmidt
    Date: 1999 Feb 08, 09:08 EST

    R.H. van Gent wrote:
    > 1) I am trying to locate an obscure reference cited in Bowditch?s
    > _American Practical Navigator: An Epitome of Navigation_ (p. 423, 1958
    > edition; vol. II, p. 546, 1977/81 edition) to a study performed by the
    > Carnegie Institute of Washington on the application of the standard
    > formula for the correction for dip:
    Dear Mr. van Gent,
    I have taken the liberty of forwarding your question on HASTRO-L to this
    list last Friday, and I received this reply which I'll simply repeat
    since you've joined us now:
    Richard Langley wrote:
    > For a good discussion of astronomical refraction, including horizontal
    > refraction, see Chapter 15 of Jean Meeus's Astronomical Algorithms.  He gives
    > as one reference a paper by Bennett: "The Calculation of Astronomical
    > Refraction in Marine Navigation" in the Journal of the (U.K.) Institute of
    > Navigation, Vol. 35, 1982, pp. 255-259.
    > -- Richard Langley
    >    Professor of Geodesy and Precision Navigation
    > On Fri, 5 Feb 1999, Tom McHugh wrote:
    > >R.H. van Gent wrote: (and others as well have stated substantially the
    > >same thing regarding dip)
    > >>
    > >> The small angle between both horizons is known as the 'dip', and can be
    > >> approximated by the following relation found in almost any astronomical
    > >> or navigational handbook:
    > >>
    > >>   dip (minutes of arc) = 0.97 sqrt(h[ft])
    > >>
    > >> with 'h' denoting the height of the observer?s eye above sea level in
    > >> feet.
    > >
    > >I think, that for beginners on the list, to avoid confusion, it
    > >would be well to state clearly that the above dip formula refers to
    > >the sea level as being one's local horizon of reference. and relates to
    > >one's
    > >vertical elevation of eyepoint above sea level. It must be pointed out
    > >that this formula will not be correct if one is on a horizontal plane
    > >at some considerable distance above sea level where the local horizon is
    > >also
    > >well above sea level. Put another way, someone living in Denver or
    > >other high plateau regions would have to calculate dip based upon
    > >height above the local horizontal plane, not referred to sea level.
    > >I am of course, referring to that portion of dip which is related to
    > >atmospheric refraction. Naturally, the eye level position above the
    > >horizontal
    > >will be the same.
    > >
    > >Even at "sea level" there would be differences in dip, as it has been
    > >determined that there are areas of the ocean's surface which are below
    > >mean sea level because of mass concentrations within the earth's crust
    > >or mantle.
    > >
    > >Tom McHugh
    > >
    > >tbmchugh@XXX.XXX
    > >
    > ===============================================================================
    >  Richard B. Langley                            E-mail: lang@XXX.XXX
    >  Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
    >  Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
    >  University of New Brunswick                   Fax:      +1 506 453-4943
    >  Fredericton, N.B., Canada  E3B 5A3
    >      Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
    > ===============================================================================
      Thomas Schmidt                    e-mail  : schmidt@XXX.XXX
    =-=  TO UNSUBSCRIBE, send this message to majordomo@XXX.XXX:     =-=
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